A hybrid car or an electric car mounts thereon a large-sized battery in order to allow itself to travel, and further, mounts thereon an air cooling blower serving as a motor provided with an impeller, as a cooling means for a circuit equipped with the battery. Cooling performance (the flow rate of air) that is required at that time is determined according to a vehicular traveling status such as a current flowing in the circuit. Therefore, the blower needs to be controlled in such a manner as to satisfy the required cooling performance. The variations of the cooling performance depend upon the blower, and particularly, greatly depend upon the variations of the rotational speed of the impeller.
As a consequence, in the case where it is assumed that the variations of the rotational speed are large so that the required cooling performance cannot be satisfied, the variations of the rotational speed have been conventionally reduced by actuating the impeller at a higher rotational speed or increasing the precision of an operational clock of a unit (i.e., a microcomputer) for measuring the rotational speed at a design stage in advance. In the meantime, means for correcting the operational clock has included a method for acquiring a clock error from a control IC via serial communications (see, for example, Patent Literature 1).
However, the above-described conventional technique needs to largely set an allowable current or heat radiation amount in a motor driving circuit so as to achieve a higher rotational speed. Therefore, a blower unit including the circuit in the conventional technique has been undesirably increased in size. In the case where the car mounts thereon a highly precise clock or a serial communication circuit, there is a problem to be solved of an increase in cost accordingly.